You can't because it's against the Fair employment act. And, though I'm a Christian, it SHOULD be. If you can discriminate against hiring people who are not Christians, non Christians can discriminate against ME. Now I was military. This is one of of those rights I'd fight and die for. To have a country free from discrimination when it comes to the basic need to support your family.
This country is built on the back of EVERYONE. Everyone pays taxes. Everyone has themselves and/or families to support. People should be hired based off need and skill. That's it. If you know a friend from church who is looking for a job, that's fine. But it is ILLEGAL to ask what their religion is during the interview regardless of what state you live in. And, it should be.
Edit: To answer your update.
You won't find a Christian temp agency because it would be illegal to open one, because the very use of one would be a violation of the Fair Employment act, and Equal Opportunity laws. There are certain jobs that require ordination, which not only requires for you to be ordained but registered with the Government. This is what allows certain positions to be reserved by the church. Other Church employees, such as music director and such are appointments through the church by the ordained. But for a business there are no such allowances.
It doesn't matter what way you go about it. I'm saying this as a favor to you, it's illegal. To purposely set out to not hire others because they are not Christians is a federal offense. If you want to post openings in your church bulletin go right a head, but to publicly announce you are looking for "Christian help"..
My friend, it can get you put in jail. If anyone you didn't hire were to connect this very internet conversation to you they could sue you, and they would win, and you could face jail time.
Also, Right to work states don't have a lot of rules about firing people. Because of this it's hard to take companies from such states to court for being fired wrongfully. Many companies seek out such sates because of this. But, these states are still held to federal law, and contrary to popular belief they CAN be taken to court for wrongfully firing an employee. But, you better have a rock solid case.
Fair employment act.: http://www.spartacus.schoolnet.co.uk/USAfairE.htm
Equal Opportunity act: http://en.wikipedia.org/wiki/Equal_Opportunity_Employment